Powders consisting of nonporous particles as well as small, granular, and porous materials of a variety of types are utilized in huge amounts by modern industry. The extent of their surface area and the size and magnitude of the pores they contain is of great significance in the way powders behave. Some powdered materials, carbon black, for example, exhibit an extremely high surface area even in small quantities. The pores in powdered catalysts, particularly zeolitic ones, can be as small as a few angstroms in diameter and nearly all of one size. Carbon black surface area per unit mass is critical in formulating rubber for long-life automobile tires, and zeolite pore size is a determining factor in which chemical reaction is promoted by a zeolite catalyst. Other examples of the critical importance of surface structure are to be found in the rate at which pharmaceutical tablets dissolve, the fusion and resulting density of ceramic bodies, and the conductance of electrical resistors.
Surface area is established by following, basically, the so-called BET technique (Brunauer, S., Emmett, P., and Teller, E., "The Adsorption of Gases in Multimolecular Layers," J. Am. Chem. Soc. 60, 309-19 (1938)), wherein a gas is adsorbed at a constant low temperature, ordinarily the temperature of liquid nitrogen, on the surface of the solid to an amount described by the well known BET equation as a monomolecular layer. The size of the gas molecule and the amount of gas, hence number of gas molecules, then establishes the magnitude of the surface. Mesopore and macropore sizes (defined, respectively, as greater than 20 .ANG. and 500 .ANG. in diameter) and volumes are computed from data relating the quantity of gas adsorbed beyond that forming the monomolecular layer and the pressure at which the adsorption occurred in conformity with the known BJH calculation procedure (Barrett, E., Joyner, L., and Halenda, P., "The Determination of Pore Volume and Area Distribution in Porous Substances-I. Computation from Nitrogen Isotherms," J. Am. Chem. Soc. 73, 373-80 (1951)).
Materials having microporous characteristics (pores less than 20 .ANG. in diameter) are evaluated in the same manner as above, that is, the materials are exposed to increasing amounts of gas and data are collected relating the quantity of gas adsorbed to the prevailing physical conditions. Extraction of material physical parameters from the data is in accordance with the known interpretation of Langmuir (J. Am. Chem. Soc. 38, 2219 seq. et. seq. (1916) and J. Am. Chem. Soc. 40, 1361 et. seq. (1918)), and the computational procedure pioneered by Polanyi (Trans. Faraday Soc. 28, 316 et. seq. (1932)), and expanded on by others, most notably Dubinin and Astakhov (Adv. Chem. Ser. 102, 69 et. seq. (1971).
By subjecting the powdered sample to incrementally increasing volumes of gas to a point at or approaching the saturation pressure of the particular gas, an adsorption isotherm may be obtained. Reversing the procedure by subjecting the sample to incremental reductions in gas pressure, also at a constant temperature, yields a desorption isotherm. The volume of gas adsorbed per unit mass of solid depends on the equilibrium pressure of the gas, the absolute temperature, and the nature of the gas and solid. An adsorption isotherm is a plot at constant temperature of the gas adsorbed per unit mass of solid versus the relative pressure P/P.sub.o, where P.sub.o is the saturation vapor pressure of the adsorbate gas. Thus, any device designed to measure surface area or pore volume data must be able to accurately determine the quantity of gas adsorbed.
Several instruments have been developed for measuring adsorption. In volumetric instruments, such as disclosed in U.S. Pat. No. 3,850,040 and U.S. Pat. No. 4,566,326, the sample is dosed with discrete amounts of an adsorbate gas from a manifold of known volume and pressure. The resulting pressure in the combined volume of the manifold and sample chamber is measured in order to determine how much of the gas is adsorbed by the sample at selected relative pressures. The volume of the manifold and of the sample compartment must be known very precisely. Thus one disadvantage of this system is that the free space, the volume within the sample compartment not occupied by the sample, must be determined precisely, because the volume of the sample varies with the quantity of sample that is chosen for analysis. Free space compensation must take into account variations in free space capacity which occur as the level of liquid nitrogen coolant surrounding the sample drops, changing the temperature profile along the sample compartment. Another problem with the volumetric system is that compensation must be made for the non-ideal behavior of the adsorbate gas. Also, the accuracy of the system depends upon absolute pressure transducers having a relatively wide pressure range. The manner of use of such transducers to determine gas volumes adsorbed requires determining the differences between two closely spaced pressures, causing effective errors and noise to increase significantly. Thus, expensive transducers with very tight noise, linearity, and error specifications must be used. Finally, the dosing sequence is tedious and lengthy.
In flowing gas sorption analyzers, such as disclosed in U.S. Pat. No. 2,960,870 and U.S. Pat. No. 3,555,912, a mixture of adsorbate and non-adsorbing gases, such as nitrogen and helium, are continuously passed over the sample. The relative proportions of the two gases in the stream are altered in order to change their relative pressures as they pass over the sample, which alters the amount of the adsorbate that will be adsorbed by the sample. Thermal conductivity detectors are placed before and after the sample to measure the change in composition of the gas stream caused by the sample, and thereby to obtain an indication of adsorption by the sample. One disadvantage of the flowing gas system is that the mixing of helium with the adsorbate limits the rate at which the adsorbate can be delivered to the sample for adsorption, and therefore slows the analysis. Also, measurements are taken at only one relative pressure of the adsorbate, with the mixture at atmospheric pressure, and the detectors are at room temperature. Therefore, the saturation pressure of the adsorbate during the run may vary with ambient conditions and is generally not accurately known. Finally, samples adsorbing large amounts of gas may require as long as 25-50 minutes to equlibrate after changes in the gas mixture, especially at low relative pressures.
A third prior system, disclosed in U.S. Pat. No. 4,762,010, delivers gas to the sample continuously at a rate claimed to be less than the equilibrium rate of adsorption with respect to the sample. The gas is released from a bulb of known volume through a leak valve into a sample compartment of known free space, and the pressure within the supply bulb and the sample compartment are both measured repetitively. The change in pressure in the bulb provides an indication of the amount of gas admitted to the sample compartment, which together with the change in pressure in the sample compartment can be used to provide an indication of the amount of gas adsorbed by the sample. Points on the adsorption isotherm can be determined continually. Problems include the slow dosing rate, the requirement for accurate knowledge of and compensation for the free space, and use of wide range absolute pressure transducers, as discussed above.
Some sorption analyzers have utilized differential pressure measurements to measure adsorption effects. For example, in the flowing gas analyzers discussed above, the difference in relative pressures of the two gases is measured by a Wheatstone bridge connecting two thermal conductivity detectors.
U.S. Pat. No. 3,349,625 discloses a gas chemisorption instrument consisting of two systems of constant and known volume, each having a reservoir of known volume, a sample chamber of known volume, valved tubing between the reservoir and chamber, and an absolute pressure measuring device attached to each reservoir. A differential pressure-measuring device is connected to measure the difference in pressure between the systems. One sample chamber contains a chemisorbing sample and the other contains a non-chemisorbing or physically adsorbing sample. The differential pressure at chemisorbing conditions is said to indicate the amount of gas chemisorbed by the chemisorbing sample.
A surface area measuring device marketed by Strohlein GmbH & Co. under the mark "AREA-meter" fills sample and comparison vessels of equal volume with nitrogen at the same pressure and ambient temperature. When brought to liquid nitrogen temperature, adsorption in the sample vessel creates a difference in pressure in the two vessels, which is measured on a differential manometer. The amount of nitrogen adsorbed by the sample can be calculated from the pressure difference and the filling pressure. U.S. Pat. No. 3,059,478 discloses a sorption system which uses a differential pressure gauge to obtain the differential pressure between a sample tube and a reference tube which appears to contain a fixed amount of nitrogen gas. A manometer system is used to supply fixed mass doses of nitrogen to the sample tube, and a plot of mass introduced to the sample versus the differential pressure is generated. This system relies on being able to deliver a sequence of precisely equal doses, and does not overcome all of the problems in the art discussed above.
Thus, there has been a need in the art for a sorption analyzer which is capable of providing precise data on the volume of gas adsorbed by a sample, is not limited in speed by the structure or operation of the apparatus, does not rely for accuracy on readings of closely spaced pressures by wide range absolute pressure transducers, does not require determination of or temperature compensation for free space in a sample compartment, and does not require compensation for non-ideal behavior of the adsorbate gas.